A roller cone earth boring bit has a bit body with typically three bit legs. A bearing shaft or pin depends downward and inward from each bit leg toward the bit body axis of rotation. A cone having cutting elements on its exterior mounts rotatably on each bearing pin. A seal gland is located at the mouth of the cone and the base of the bearing pin. A variety of seal assemblies may be mounted in the seal gland to seal lubricant in the bearing spaces and inhibit the entry of drilling fluid into the bearing spaces.
The sealing elements have to perform at least two functions, including providing an appropriate sealing force against the surface being sealed and conforming to the surfaces being sealed. These functions have to be performed for the intended service duration in the service environment. Among other things, this requires that the sealing elements resist chemical and mechanical attack by the materials being excluded and sealed and further that they resist detrimental changes in properties in their service environment.
Oilfield roller cone drill bits are required to operate in conditions of severe mechanical vibration, high pressures (frequently greater than 10,000 psi and potentially greater than 20,000 psi) and moderately high temperatures (frequently greater than 150 deg C., and potentially greater than 200 deg C.), when immersed in aqueous and/or hydrocarbon based fluids. The fluids frequently contain substantial volume fractions of potentially abrasive solid particles. The bit bearings are lubricated with grease supplied from internal reservoirs. The bearings are sealed in order to prevent the solids containing drilling fluid from entering the bearing. Typically the primary seal is placed between the rotating cone and the pin on which it rotates. Rapid bearing wear leading to premature bearing failure occurs should a seal fail in service. There are two main classes of seals in use in oilfield roller cone bits today—elastomeric and mechanical face seals.
The majority of elastomeric seals are “O” rings, but high aspect ratio (HAR) elastomeric seals are also used. Radial compression of the seal cross section provides the sealing force and the relatively soft and pliable nature of the elastomer allows it to conform quite closely to the surfaces of the glands against which it rims. The primary processes limiting the operating life of elastomeric seals are (1) abrasive wear of the sliding surfaces and (2) compression set at elevated operating temperature, causing the seal to harden and permanently deform. Both these processes cause the seal to lose its “squeeze” or sealing force. There are many patents relating to elastomeric seals, their geometry and materials.
The sealing components of mechanical face seals are typically hard metals with flat sealing surfaces that slide one over the other. One or more of the sliding surfaces may be coated with a wear resistant layer. In commercially successful metal face seals, the sealing force is provided by one or two elastomeric energizer elements forcing the sealing elements one against the other. The energizer and the separate elastomeric back-up ring, if provided, provide static sealing in addition to the dynamic seal provided by the metallic sliding surfaces. Abrasive wear of the sliding metallic surfaces can lead to seal leakage. So too can loss of sealing force arising from compression set of the elastomeric energizer. In some instances leakage may occur due to abrasive wear if the energizer slides unintentionally against its static seat. A mechanical face seal may fail prematurely if the sealing faces open temporarily during transient rocking or inward movement of the cone on the bearing pin. If the faces open, solids containing drilling fluid may enter the seal and promote wear of the sealing surfaces. The failure mode is likely to become more prevalent if the energizer does not respond sufficiently rapidly to the transient motion of the cone, for instance if it possesses high internal damping. There are many patents relating to mechanical face seals for oilfield roller cone bits and for other applications. Some of these relate to the use of metallic springs to provide the sealing force.